This invention relates to photogeneration of active formate decomposition catalysts containing manganese. More specifically, this invention is directed to producing hydrogen and carbon dioxide from formate salts and water by photogenerating an active formate decomposition catalyst. Additionally, the process may be expanded to generate formate salts from carbon monoxide and base. These two reactions when combined comprise the water-gas shift reaction.
Carbon monoxide is readily produced by the partial oxidation of a wide variety of carbon containing substances including coal, petroleum, and biomass. Carbon monoxide is not only a valuable feedstock for a variety of industrial synthesis processes but also can be combined with water to produce hydrogen which is another chemical feedstock of great value. This chemical reaction is known in the art as the water-gas shift reaction.
Current technology utilizes steam as a reactant in the water-gas shift reaction as shown in equation (1): EQU CO(g)+H.sub.2 O(g).revreaction.H.sub.2 (g)+CO.sub.2 (g) (1)
This reaction is generally carried out in two stages with the first stage utilizing an iron oxide-chromium oxide catalyst operating at 315.degree. C.-485.degree. C. followed by a second stage which uses a zinc oxide-copper oxide catalyst system operating at lower temperatures, 175.degree. C.-350.degree. C. The incorporation of a second stage operating at a lower temperature is advantageous because the reaction is exothermic so that lower temperatures favor hydrogen formation.
In recent years considerable attention has been directed towards conducting the water-gas shift reaction under still milder conditions at temperatures between 100.degree. C. and 200.degree. C. with water present in a condensed state. This reaction is shown in equation (2): EQU CO(g)+H.sub.2 O(l).revreaction.H.sub.2 (g)+CO.sub.2 (g) (2)
While highly favored from a free energy standpoint, this reaction unlike reaction (1) is mildly endothermic; thus, not only can a savings in energy be realized by operating at these lower temperatures, but in addition the mild endothermicity of equation (2) offers an advantage from an engineering standpoint in that any isothermal reactor using condensed water as a reactant will have reduced cooling demands in comparison to conventional systems. With the reactant water being present in a condensed state, homogeneous catalysts afford a convenient means for accelerating this reaction. Since the products, hydrogen and carbon dioxide, are gases, the separation of catalyst from product poses no problem. Finally, any commercial operation using condensed water and homogeneous catalysts under such mild conditions is likely to be inexpensive to construct, thus requiring low capital investment.
U.S. Pat. No. 4,372,833 discloses that Group Vlb hexacarbonyl compounds, Cr(CO).sub.6, Mo(CO).sub.6, and W(CO).sub.6, when irradiated by light become active catalysts which greatly accelerate the chemical reaction whereby formate ion decomposes in the presence of water to form hydrogen and carbon dioxide according to equation (3): EQU HCO.sub.2.sup.- +H.sub.2 O(l).revreaction.H.sub.2 (g)+CO.sub.2 (g)+OH.sup.-( 3)
Formate ion, on the other hand, is readily produced by a reaction between carbon monoxide and hydroxide ion according to reaction (4): EQU CO+OH.sup.- .fwdarw.HCO.sub.2.sup.- ( 4)
The reactions of equations (3) and (4), when carried out together, constitute the water-gas shift reaction. See equation (2). The reaction of equation (4) proceeds very rapidly so that catalysis of the formate decomposition reaction can effect a marked acceleration of the overall water-gas shift reaction. See equation (2). In cases where the reaction proceeds in two steps, the reaction of equation (4) is followed by the reaction of equation (3).
A subsequent survey of transition-metal carbonyl compounds has shown that cyclopentadienylmanganesetricarbonyl and methylcyclopentadienylmanganesetricarbonyl when photolyzed are both capable of greatly accelerating the rate of decomposition of formate ion in the presence of water. The element manganese belongs to Group VIIB of the periodic table, and there is no reason to expect elements of this group to exhibit similar chemical properties to those of Group VIB: Cr, Mo, and W. Nevertheless, the rates of decomposition of formate ion induced by methylcyclopentadienylmanganesetricarbonyl are approximately double those achieved using Cr(CO).sub.6 under equivalent conditions. The reaction rates are thus significantly improved over rates for similar reactions known in the art. An exhaustive series of experiments have not been performed in order to delineate the mechanism by which this catalyst operates; therefore, no basis exists for comparing the mode of action of cyclopentadienylmanganesetricarbonyl and methylcyclopentadienylmanganesetricarbonyl with that disclosed previously in U.S. Pat. No. 4,372,833 for Group VIb hexacarbonyls. The total data available for these systems are listed in the following Examples and Tables I-VII of the Description of the Preferred Embodiments.
It is the object of the present invention to produce hydrogen from formate ion and water by photogenerating an active formate decomposition catalyst containing manganese. Additionally, the process may be expanded to generate formate ion from carbon monoxide and hydroxide ion, the sum of the two reactions being the water gas shift reaction. It is a further object to provide a novel catalytic process for producing hydrogen at relatively low temperatures and otherwise mild conditions wherein the cost of compounds cyclopentadienylmanganesetricarbonyl and methylcyclopentadienylmanganesetricarbonyl are very modest wherein compound methylcyclopentadienylmanganesetricarbonyl is readily available and wherein the reaction rates are significantly improved over rates for similar reactions known in the art.
These and other objects, aspects and advantages of this invention will become apparent from a consideration of the accompanying specification and claims.